scholarly journals Three-phase chemical models of dense interstellar clouds: gas, dust particle mantles and dust particle surfaces

1993 ◽  
Vol 263 (3) ◽  
pp. 589-606 ◽  
Author(s):  
T. I. Hasegawa ◽  
E. Herbst
Keyword(s):  
Dust Particle ◽  
Interstellar Clouds ◽  
Chemical Models ◽  
Three Phase ◽  
Phase Chemical

scholarly journals A Revised Description of the Cosmic Ray Induced Desorption of Interstellar Ices

2021 ◽  
Vol 922 (2) ◽  
pp. 126
Author(s):  
Olli Sipilä ◽  
Kedron Silsbee ◽  
Paola Caselli
Keyword(s):  
Gas Phase ◽  
Cosmic Ray ◽  
Equilibrium Temperature ◽  
Cooling Time ◽  
Transient Heating ◽  
Two Phase ◽  
Chemical Models ◽  
Three Phase ◽  
Prestellar Cores ◽  
Phase Chemical

Abstract Nonthermal desorption of ices on interstellar grains is required to explain observations of molecules that are not synthesized efficiently in the gas phase in cold dense clouds. Perhaps the most important nonthermal desorption mechanism is one induced by cosmic rays (CRs), which, when passing through a grain, heat it transiently to a high temperature—the grain cools back to its original equilibrium temperature via the (partial) sublimation of the ice. Current cosmic ray induced desorption (CRD) models assume a fixed grain cooling time. In this work, we present a revised description of CRD in which the desorption efficiency depends dynamically on the ice content. We apply the revised desorption scheme to two-phase and three-phase chemical models in physical conditions corresponding to starless and prestellar cores, and to molecular cloud envelopes. We find that, inside starless and prestellar cores, introducing dynamic CRD can decrease gas-phase abundances by up to an order of magnitude in two-phase chemical models. In three-phase chemical models, our model produces results very similar to those of the static cooling scheme—when only one monolayer of ice is considered active. Ice abundances are generally insensitive to variations in the grain cooling time. Further improved CRD models need to take into account additional effects in the transient heating of the grains—introduced, for example, by the adoption of a spectrum of CR energies.

scholarly journals The Gas-phase Chemical Evolution of Dark Clouds

1992 ◽  
Vol 45 (4) ◽  
pp. 451
Author(s):  
RPA Bettens
Keyword(s):  
Gas Phase ◽  
Chemical Species ◽  
Dynamical Evolution ◽  
Constant Density ◽  
Dark Cloud ◽  
Dark Clouds ◽  
Chemical Models ◽  
Almost All ◽  
Present Understanding ◽  
Phase Chemical

A rich chemistry exists within dark clouds. In the most chemically studied dark cloud, Taurus molecular cloud one (TMC-l), more than 40 molecules have been detected. In this paper I look at the current isochoric, i.e. constant density, isothermal time-dependent gas-phase chemical models of dark clouds such as TMC-l and very briefly outline the present understanding of the chemistry of these objects. The above chemical models agree very well with the observed abundances of almost all chemical species at times earlier than steady state, i.e. earlier than thirty million years. However, the models are fraught with uncertainty and are not physically realistic representations of the full dynamical evolution of dark clouds from a more diffuse state. Nevertheless the agreement with observation is striking.

scholarly journals Evolutionary Models of Interstellar Chemistry

1992 ◽  
Vol 150 ◽  
pp. 205-210
Author(s):  
Sheo S. Prasad
Keyword(s):  
Gas Phase ◽  
Chemical Processes ◽  
Evolutionary Models ◽  
Evolutionary Perspective ◽  
Interstellar Chemistry ◽  
Interstellar Clouds ◽  
Dynamical Equilibrium ◽  
Chemical Models ◽  
Gas Phase Molecules ◽  
Fixed Condition

Evolutionary chemical models are ultimately unavoidable for a full understanding of interstellar clouds. They include not only the chemical processes but also the dynamical processes by which the modeled object came to be the way it is. From an evolutionary perspective, dark cores may be ephemeral objects and dynamical equilibrium an exception rather than norm. Evolutionary models have numerous advantages over “classical” fixed condition equilibrium models. They have the potential to provide more elegant explanations for the observed inter-cloud and intra-cloud chemical differences. The problem of the depletion of gas phase molecules by condensation onto the grain may also be less serious in evolutionary models. Hence, these models should be actively developed.

scholarly journals Chemistry in Dense Interstellar Clouds – Data Requirements

1989 ◽  
Vol 8 ◽  
pp. 369-374 ◽  
Author(s):  
T. J. Millar
Keyword(s):  
Experimental Data ◽  
Observational Studies ◽  
Rate Coefficients ◽  
Interstellar Clouds ◽  
Chemical Models ◽  
Data Requirements

ABSTRACTChemical models of dense interstellar clouds are reviewed with particular emphasis on recent results. The need for theoretical and experimental data on rate coefficients is pointed out and some observational studies are suggested.

scholarly journals First detection of H2S in a protoplanetary disk

2018 ◽  
Vol 616 ◽  
pp. L5 ◽  
Author(s):  
N. T. Phuong ◽  
E. Chapillon ◽  
L. Majumdar ◽  
A. Dutrey ◽  
S. Guilloteau ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Reasonable Agreement ◽  
Protoplanetary Disks ◽  
Protoplanetary Disk ◽  
Circumstellar Disk ◽  
T Tauri ◽  
Upper Limits ◽  
Three Phase ◽  
Chemical Code ◽  
Phase Chemical

Context. Studying molecular species in protoplanetary disks is very useful to characterize the properties of these objects, which are the site of planet formation. Aims. We attempt to constrain the chemistry of S-bearing molecules in the cold parts of circumstellar disk of GG Tau A. Methods. We searched for H2S, CS, SO, and SO2 in the dense disk around GG Tau A with the NOrthem Extended Millimeter Array (NOEMA) interferometer. We analyzed our data using the radiative transfer code DiskFit and the three-phase chemical model Nautilus. Results. We detected H2S emission from the dense and cold ring orbiting around GG Tau A. This is the first detection of H2S in a protoplanetary disk. We also detected HCO+, H13CO+, and DCO+ in the disk. Upper limits for other molecules, CCS, SO2, SO, HC3N, and c-C3H2 are also obtained. The observed DCO+/HCO+ ratio is similar to those in other disks. The observed column densities, derived using our radiative transfer code DiskFit, are then compared with those from our chemical code Nautilus. The column densities are in reasonable agreement for DCO+, CS, CCS, and SO2. For H2S and SO, our predicted vertical integrated column densities are more than a factor of 10 higher than the measured values. Conclusions. Our results reinforce the hypothesis that only a strong sulfur depletion may explain the low observed H2S column density in the disk. The H2S detection in GG Tau A is most likely linked to the much larger mass of this disk compared to that in other T Tauri systems.

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